It is conventional to use a plurality of individually movable flaps to form nozzles at the exhaust end of large turbofan aircraft engines. The flaps are typically pushed or pulled in an omnidirectional sense, the movement being guided by two or more tracks within which a sliding element disposed on the flap will translate so as to define the flap's motion. An example is patent publication US 2011/0297758. In that publication, a slider block slides within tracks, but it is also known to utilize rollers which roll within tracks. The blocks or rollers and tracks provide a guided constraint when the flaps are commanded to move in response to a change of the nozzle area ratio or when nozzle vectoring (steering of the aircraft) is required. However, excessive wear of the tracks and the elements that slide within them has been noted.